A pop quiz on some engine tests

Subject - a 200TQ, built in 5/88, w/a virgin engine.
Bentley sez compression should be 123psi to 144psi for a brand new MC
engine (my guess is that the lower value is for my single knock MC
w/compression ratio 7.6:1 and the higher value is for a late dual knock
MC w/compression ratio 8.4:1 ). Wear limit is 92psi, max delta between
the cylinders should be no more than 44psi.
The leak down scale on the gauge that I used is calibrated as such:
Set initial pressure: 0-10%
Low leak: 10-40%
Moderate leak: 40-70%
High leak: 70-100%
---------------------------------------------------------------------
Actual test values before the head removal=>
Compression:
#1 - 120psi
#2 - 122psi
#3 - 115psi
#4 - 118psi
#5 - 115psi
(not bad for a 10y/o car!)
Leak down test:
#1 - 20%
#2 - 18%
#3 - 18%
#4 - 30%
#5 - 25%
(acceptable)
Removed the head. EM was cracked in 3 places. Replaced it with a new
2-piece unit. Lapped the valves. Checked their radial play in the guides
-were OK. Scraped all C deposits from piston tops, cylinder walls,
combustion chamber. Replaced all seals, put the engine back together.
Actual test values after the head removal=>
Compression:
#1 - 115psi
#2 - 115psi
#3 - 115psi
#4 - 115psi
#5 - 115psi
(dead nuts uniformal, albeit somewhat lower)
Leak down test:
#1 - 5%
#2 - 5%
#3 - 5%
#4 - 10%
#5 - 5%
(wow! better than "low")
Now the question: how come that after doing the head work and lapping
the valves the compression figures have dropped while the leak down test
shows a great improvement, meaning that the valves are lapped very well
and that the rings seal very tight? Why is such contradiction? A damn
mystery!
It took me a while to figure out why... I'll post the answer tomorrow.
Good night.
:)
--
Igor Kessel
'89 200TQ -- 18psi (TAP)
'98 A4TQ -- nothing to declare
Philadelphia, PA
USA